- carried out his orders blindly and without thinking ( see http://en.wikipedia.org/wiki/Battles_of_Rzhev where he blindly sent several hundred thousand troops into fortified German position - if you send your forces up the same road for two months straight without gaining any advances, wouldn't you at least try to find an alternate route that the enemy doesn't expect you to use? ) ,
- was very hungry for fame and glory (I can't find references now, but it was his initial reports to higher command during the first few days after 1941-06-22 that led Stalin to believe the German invasion was already mostly contained and delayed Russian defense deployments. In addition, Stalin demoted Zhukov later, citing "attributes others' achievements to himself" as a significant reason.), and
- was generally vain, sadistic and incompetent. Several high ranking officials and civilians reported his attitude towards his subordinates as nothing less than disgusting.
- He was also in charge of Totsk nuclear testing ( http://www.johnstonsarchive.net/nuclear/radevents/1954USSR1.... ), where he had over 40k soldiers split into two teams and engage each other moments after a nuke had been detonated above the battlefield.
Also, he had nothing to do with the German defeat at Stalingrad - he was still too busy headbutting Rzhev: http://en.wikipedia.org/wiki/Battle_for_Velikiye_Luki_(1943)
For more details, I suggest looking up Viktor Suvorov's "Shadow of Victory". And other Suvorov's books for a different view of WWII in general.